Coffee Cake

Method

1. mix3 min
   streusel first, rubbed to clumps
   brown sugar + cinnamon + 3/4 cup of the flour + 1/4 cup of the butter (cold) + walnuts rubbed to damp clumps — made first because it needs no care, and the batter that comes next does
2. whisk1 min
   remaining flour + baking powder + soda + salt — both leaveners on purpose: the powder lifts, the soda neutralizes the sour cream's acid and browns the crumb
3. mix4 min
   medium-high, cream the remaining butter and sugar
   butter + sugar to pale and fluffy, then eggs one at a time + vanilla — the standard creaming contract: the air cut here is the rise, and cold eggs would curdle what the beater just built
4. fold3 min
   flour and sour cream alternating, minimal strokes
   dry and sour cream in alternating additions ending on flour — the batter is thick, nearly frosting-like, and that is correct; it has to be stiff enough to hold a sugar ribbon aloft
5. prep4 min
   half the batter, half the streusel, repeat
   half the batter spread in the buttered pan, half the streusel scattered, the rest of the batter dolloped and smoothed over (it resists — persist), remaining streusel on top — the buried ribbon melts into a cinnamon stratum while the surface layer bakes to crunch: one mixture, two fates
6. bake45 min
   350°F, tester clean of batter
   until golden and a tester finds moist crumbs but no wet batter — the streusel top browns ahead of the crumb, so trust the tester over the color; tent loosely with foil if the walnuts threaten to scorch in the last ten
7. rest15 min
   in the pan
   before cutting — warm is the canonical serving temperature, when the interior ribbon is still molten; with coffee, obviously